Not sure how old you are, but imagine this... Your child is struggling with Physics A level (and a B minimum is critical), so you get a tutor from one of the big (assume reputable) agencies. Due to clashes, you never seem to have the same one turn up for your kid's lesson (at £35/hr). Would you let that get to 10 instructors? Would you believe the BS they'd give that, "everything is standardised, so don't worry..."? It won't hurt to tell the school that the next time you get a stand-in, you're off. If they seem not to care, go anyway.
CG